FWIW, we forcibly remove stuff from /tmp after 3 days via a simple perl script that does time conversions etc. Putting that policy in place + enforcement deals with the brokenness that is Pig and just general users being bad.
But right now, instead of leaving this cruft in /tmp, distcp is likely spreading zero length files throughout your entire file system. Luckily, distcp uses a fairly easily recognizable and random filename. With this patch, this is now in /tmp and easily dealt a rm.
Like most of my patches, I expect this patch has 0 chance of actually getting committed and is only here for others that want a fix.